This was actually what inspired the series. It was Rarity's Hazmat suit in "Bats!" (4x07) that got the ball rolling. Some of you guys said geology, which I didn't think of, but I like Rarity in a Hazmat suit. But really, geology would've been better. I might do an addendum. The Ponies of Science series: The Mane Six Representing Six Non-Exclusive Fields of Science!

This is a pretty textbook reason why most hazardous materials suits are kept at positive pressure and the work spaces in the labs are kept at negative pressure- so if there are any holes the contaminants are blown away.

Twilight Sparkle: She is a chemist because she likes to do experiment(she has a basement for that, see the episode "Pinkie Sense" or fan-made episode "Double Rainboom").

Fluttershy: She's a veterinarian because she took care her pets in her cabin.(actually, this one is obvious)

Rainbow Dash: Although this is related to pilot(no planes in MLP world), but she likes to fly fast(like a jet? You could said that). So she needs aeronautics to achieve this speed.

Pinkie Pie: As a physicist(I believe that's true)? Because she like to hopping around and shoot confetti out from her party cannon.

Applejack: As a botanist. Because she took care of her apple trees(they are plants right?).

Rarity: I'm not sure why she's a microbiologist(microbiology=study microorganisms), because she likes fashion & making clothes. Not study about bacteria and stuff.For my opinion, she's suitable to study about geology.